Human Physiology, 14th edition (2016)

26 Chapter 2

2 electrons. If an atom has more than 2 electrons (as do all
atoms except hydrogen and helium), the additional electrons
must occupy shells that are more distant from the nucleus. The
second shell can contain a maximum of 8 electrons, and higher
shells can contain still more electrons that possess more energy
the farther they are from the nucleus. Most elements of bio-
logical significance (other than hydrogen), however, require
8 electrons to complete the outermost shell. The shells are
filled from the innermost outward. Carbon, with 6 electrons,
has 2 electrons in its first shell and 4 electrons in its second
shell ( fig. 2.1 ).
It is always the electrons in the outermost shell, if this
shell is incomplete, that participate in chemical reactions and
form chemical bonds. These outermost electrons are known as
the valence electrons of the atom.

Isotopes

A particular atom with a given number of protons in its nucleus
may exist in several forms that differ from one another in their
number of neutrons. The atomic number of these forms is thus
the same, but their atomic mass is different. These different
forms are called isotopes. All of the isotopic forms of a given
atom are included in the term chemical element. The element
hydrogen, for example, has three isotopes. The most common
of these has a nucleus consisting of only 1 proton. Another iso-
tope of hydrogen (called deuterium ) has 1 proton and 1 neu-
tron in the nucleus, whereas the third isotope ( tritium ) has
1 proton and 2 neutrons. Tritium is a radioactive isotope that is
commonly used in physiological research and in many clinical
laboratory procedures.

Chemical Bonds, Molecules,

and Ionic Compounds

Molecules are formed through interaction of the valence

electrons between two or more atoms. These interactions,

such as the sharing of electrons, produce chemical bonds

( fig. 2.2 ). The number of bonds that each atom can have is

determined by the number of electrons needed to complete

the outermost shell. Hydrogen, for example, must obtain

only 1 more electron—and can thus form only one chemical

bond—to complete the first shell of 2 electrons. Carbon, by

contrast, must obtain 4 more electrons—and can thus form

four chemical bonds—to complete the second shell of 8 elec-

trons ( fig. 2.3 , left ).

Covalent Bonds

Covalent bonds result when atoms share their valence

electrons. Covalent bonds that are formed between iden-

tical atoms, as in oxygen gas (O 2 ) and hydrogen gas (H 2 ),

are the strongest because their electrons are equally shared.

Because the electrons are equally distributed between the

2 atoms, these molecules are said to be nonpolar and the

bonds between them are nonpolar covalent bonds. Such

bonds are also important in living organisms. The unique

nature of carbon atoms and the organic molecules formed

through covalent bonds between carbon atoms provides the

chemical foundation of life.

When covalent bonds are formed between two different

atoms, the electrons may be pulled more toward one atom than

the other. The end of the molecule toward which the electrons

are pulled is electrically negative compared to the other end.

Such a molecule is said to be polar (has a positive and nega-

tive “pole”). Atoms of oxygen, nitrogen, and phosphorus have

Figure 2.1 Diagrams of the hydrogen and carbon
atoms. On the left, the electron shells are represented by shaded
spheres indicating probable positions of the electrons. On the
right, the shells are represented by concentric circles.

Proton Neutron Electron

Hydrogen

1 proton

1 electron

Carbon

6 protons

6 neutrons

6 electrons

Figure 2.2 A hydrogen molecule showing the

covalent bonds between hydrogen atoms. These bonds are

formed by the equal sharing of electrons.

H 2